Unexplained hyponatremia: seek and you will find.

Hoorn, Ewout J; Hotho, Daphne; Hassing, Robert Jan; Zietse, Robert

BACKGROUND Hyponatremia is a common diagnostic challenge. METHODS An index case is presented to discuss the diagnostic approach to chronic and unexplained hyponatremia. RESULTS The index case concerns a 60-year-old man with chronic hepatitis C and previous alcohol use who was referred because of weight loss, poor dietary intake, dizzy spells, and unexplained hyponatremia (serum sodium 124-129 mmol/l). A low urine sodium concentration (20 mmol/l) and a low fractional sodium excretion (0.07%) were observed repeatedly, while urine osmolality was high (>400 mosm/kg). The central questions in this case are: what is the differential diagnosis, which tests are needed to confirm or exclude a diagnosis, and how would you proceed if no obvious cause is found? CONCLUSIONS The diagnosis of this case of unexplained hyponatremia was unexpected, but important because it was treatable. The challenges and caveats of the diagnostic approach to hyponatremia are discussed. A diagnostic algorithm to guide clinicians who are confronted with similar cases is presented.